Many plastic articles suffer from poor environmental stress crack resistance. Typically on exposure to cleaners or oily or fatty materials the plastic suffers a significant loss in physical properties and may spontaneously develop cracks. "Environmentally friendly" is a term which is gaining wide use in industry and advertising. Generally it connotes a product having a good compatibility with the environment. Unfortunately "environmentally friendly" is much like beauty. It is in the eye of the beholder or a matter of perspective. While sodium carbonate based cleaners are environmentally friendly in the sense that they do not rely on organic solvents of strong bases or acids they have one drawback. They tend to be very hard on plastics and particularly on copolymers or alloys containing polymers of vinyl aromatic monomers and alkyl methacrylates.
In concurrent research Novacor Chemicals Inc. developed a polymer alloy which has outstanding toughness. There was nothing in the initial program which indicated that the alloy had any improved chemical properties such as environmental stress crack resistance to cleaners including those based on sodium bicarbonate. Applicants have discovered this unique property of the polymer.
The above noted polymer is the subject matter of U.S. patent application Ser. No. 654,058 filed Feb. 12, 1991.
The following is a list of references which applicants feel may be relevant to the composition of the new polymer alloy of the present invention.
Chemical Abstracts 109:171278v [Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1988, 29(2), 180-1] discloses that physical blends of block SBR (styrene content: 75.2 weight percent) and a copolymer of methyl methacrylate and styrene are partially compatible. The morphological and mechanical properties of the blend are strongly dependent on the molding/mixing process used to produce the blend. The SBR could only be toughened when the ideal morphology of the blend was obtained.
Chemical Abstracts 106:85901z [JP 61/200,151] discloses thermoplastic compositions comprising from 5 to 95 weight percent of a block copolymer and from 95 to 5 weight percent of a thermoplastic polymer (e.g. ABS). The block copolymer is derived from coupling an aromatic vinyl polymer block with an aromatic vinyl monomer-conjugated diene copolymer block. The subject thermoplastic compositions apparently have improved impact strength.
Chemical Abstracts 112:8285y [JP 61/291,610] discloses the use of "impact modifiers" for thermoplastic polymers. The modifiers are basically similar to the block copolymer disclosed by Chemical Abstracts 106:85901z.
Chemical Abstracts 99:196070j [JP 58/122,954] discloses improvement of anisotropy of a styrene hydrocarbon-conjugated diene block copolymer by blending it with a styrene-methacrylic ester copolymer. Such a blend, when extruded to form a molded plate, apparently possesses superior physical properties.
Chemical Abstracts 111:8522c [JP 01/45614] discloses high-impact styrene polymer compositions comprises a major amount of a styrene (50 to 90 weight percent)-conjugated diene (50 to 10 weight percent) block copolymer and a minor amount of a styrene-based polymer (e.g. styrene-methyl methacrylate copolymer).
The above art all discusses the physical or mechanical properties of similar alloys. However, none of the art suggests that such alloys could have any improved chemical properties.
It would be desirable to have a composition which possesses a combination of improved properties, including improved impact strength. Typically the alloys of the present invention should have a notched Izod impact strength of greater than about 0.5, preferably greater than 1.0 ft.lb./in. In a particularly preferred aspect of the present invention, the alloys may have a haze of less than 10, most preferably less than 5. It is also desirable to have such polymers which retain at least 80, more preferably 85% of their flexural, tensile, and Izod impact properties after exposure for 1 hour to a sodium bicarbonate based cleaner or a 75% solution of sodium bicarbonate. Most preferably the composition will not show visible stress cracks (small cracks) on exposure to sodium bicarbonate based cleaners or a 75% solution of sodium bicarbonate. Polymers or polymer alloys having the above properties are referred to in this specification as having good environmental stress crack resistance (ESCR).